A heat-transfer tube configuring a heat exchanger is heated by combustion exhaust gas of a gas burner to thereby raise a temperature of an inside fluid. There has been known a technique of inserting, into the heat-transfer tube, the turbulence member configured to generate turbulence in the fluid inside the heat-transfer tube in order to suppress local boiling of the fluid inside the heat-transfer tube and to promote heat exchange, thereby increasing heat efficiency (For example, Japanese Unexamined Patent Publication No. H11-108458 A).
As the turbulence member, for example, as shown in FIG. 8, there is a turbulence member having a configuration in which by applying cut-and-raising-and-bending work to a flat plate member 3, cut-and-raised holes 30a, 30b, 30c, and cut-and-raised pieces 31a, 31b, 31c are formed, and the plurality of cut-and-raised pieces 31a, 31b, 31c are projected in both front and back surfaces of the flat plate member 3. In this turbulence member, the fluid flowing inside the heat-transfer tube hits the cut-and-raised pieces 31a, 31b, 31c, and thereby, as indicated by arrow, the turbulence of the fluid can be promoted, so that the local boiling can be efficiently suppressed.
Moreover, in the above-described turbulence member, in order that the turbulence member may not unexpectedly rotate in a state set at a predetermined position inside the heat-transfer tube 4 having an elliptical cross-sectional shape, as shown in FIG. 9, both side edges of the flat plate member 3 along a flow passage direction are bent and these bent portions 32 are brought into contact with a tube wall inner circumferential surface 40 of the heat-transfer tube 4 to thereby function as rotation preventing means of the turbulence member (For example, Japanese Unexamined Patent Publication No. S62-13958 A).
Each of the bent portions 32 formed in the flat plate member 3 cannot adhere to the tube wall inner circumferential surface 40 made of a curved surface of the heat-transfer tube 4 having a circular or elliptical cross-sectional shape, so that a slight gap 30 is caused between each of the bent portions 32 and the tube wall inner circumferential surface 40.
Since this gap 30 is very small, a flow passage resistance is large, so that the fluid hardly flows, and stagnation of the fluid is caused inside the gap 30. Further, since when a foreign substance such as dust and the like flows into the gap 30 and deposits there, the fluid further stagnates inside the gap 30, there is a possibility that gap corrosion is caused in the heat-transfer tube 4 from a portion of the gap 30.